Conventional nanopatterning techniques have mainly been made to develop processes capable of forming more precise and finer patterns using a photolithography technique. However, this technique has already reached a limitation of technology due to a resolution limitation resulting from wavelengths of light. Accordingly, a self-assembled structure control process of block copolymers is emerging as a new alternative method for nanolithography.
The block copolymer is a polymer in which blocks having different chemical compositions and structures are linked together with forming covalent bonds, where blocks having different characteristics in one molecule result in microphase separation, while the characteristics to cause phase separation are offset by covalent bonds, and eventually form a nanostructure through a specific shape (spherical, cylindrical, layered, etc.) periodic arrangement. This provides an optimal system capable of forming a fine pattern with high resolution. In addition, since the nanostructure formed by the block copolymer can be also controlled in the shape as well as the size and selected for chemical properties, they have an advantage in being applied to a nanotechnology field.
In order to apply the self-assembled structure of the block copolymer to nanolithography, it is necessary to selectively remove one block polymer through various etching processes. The wet etching process does not require high-vacuum equipments as in a dry process, and thus has an advantage that it can be applied to a large-area substrate through an inexpensive and simple process.
However, despite these advantages, in the case of the conventional wet etching method, it is possible to perform a selective etching without defects in a thin film having a thickness of up to one period in the self-assembled structure of the block copolymer (see FIG. 2), but if the thickness of the block copolymer thin film exceeds one period of the self-assembly structure, there is a problem that the porous defects appear as in FIG. 1.
For example, in the case of the best known polystyrene-block-polymethyl methacrylate (PS-b-PMMA) block copolymer, when the vertically oriented cylinder self-assembled structure is etched, it can be confirmed that the PMMA block is selectively removed by a simple process of dipping it in acetic acid and then washing it with distilled water, but it can be confirmed that the closer to the bottom of the thin film, that is, to the substrate, the porous shape defects appear (see FIG. 1). As a result, a cylinder hole pattern vertically formed on the substrate could not be obtained, and thus there was a limit to obtain a vertical pore structure having a high aspect ratio through the conventional wet etching process conditions.